Extend Zstandard SmallestSize test fallback to Apple mobile platforms

[Copilot]

ZstandardStreamUnitTests intermittently throws OutOfMemoryException on physical tvOS devices (~2 GB RAM) because CompressionLevel.SmallestSize maps to Zstd's max-quality mode, which needs 800+ MB of working memory.

Root cause

The test helper already downgrades SmallestSize → Optimal for 32-bit processes for the same memory reason, but the check didn't account for memory-constrained Apple mobile devices (iOS/tvOS/MacCatalyst).

Fix

• Extended the fallback condition in CompressionStreamUnitTests.Zstandard.cs to also trigger when PlatformDetection.IsAppleMobile is true, covering both CreateStream(Stream, CompressionLevel) overrides.

if ((PlatformDetection.Is32BitProcess || PlatformDetection.IsAppleMobile) && level == CompressionLevel.SmallestSize)
{
    level = CompressionLevel.Optimal;
}

This is a test-only change; no product code is affected.

• Fixes [ci-scan] Test failure: ZstandardStreamUnitTests OutOfMemoryException on tvOS CoreCLR #132459

[azure-pipelines]

Azure Pipelines:
16 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/runtime-infrastructure
See info in area-owners.md if you want to be subscribed.

[azure-pipelines]

Azure Pipelines:
Successfully started running 3 pipeline(s).
13 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[Copilot]

Pull request overview

Updates the Zstandard test stream factory in System.IO.Compression tests to avoid using CompressionLevel.SmallestSize on platforms where it is known to require very large working memory, by downgrading it to CompressionLevel.Optimal when running on Apple mobile platforms (in addition to the existing 32-bit fallback).

Changes:

• Extend the existing SmallestSize -> Optimal fallback to also apply on PlatformDetection.IsAppleMobile.
• Apply the same fallback consistently across both CreateStream(Stream, CompressionLevel) overloads.
• Update the accompanying comment to document the Apple mobile rationale.